Abstract

We studied the population sizes and substrate incorporationpatterns of three phylogenetic groups ofBetaproteobacteria in a coastal subtropical lagoonthat is characterized by a sharp transition from humicfreshwater to turbid brackish water. Various cellularprocesses were addressed by short-term incubationswith four radiolabelled compounds and microautoradiographicassessment of substrate incorporation.Group-specific differences in the abundances and therespective physiological state of the three populationswere observed upon transfer from the humicrichcompartment to the main body of the lagoon(estimated at 1–2 days). Members of the clade B ofPolynucleobacter (PnecB) experienced only an insignificantchange in cell numbers, but displayed ageneral metabolic downshift, carbon metabolism(glucose incorporation) being most affected. By contrast,bacteria from the closely related PolynucleobacterC clade (PnecC) clearly differed in totalabundances and in the numbers of DNA-synthesizingor glucose incorporating cells. At the same time,PnecC bacteria maintained comparable levels ofprotein synthesis (leucine uptake) in both lagooncompartments, and the proportion of cells incorporatingN-acetylglucosamine was even higher in the mainbody of the lagoon. Members of the R-BT lineageshowed little changes in cell numbers, DNA synthesisand carbon metabolism. Altogether, the observed patternsof substrate metabolism suggest that differentbacterial populations in the lagoon undergo specificphysiological adjustments in response to changingenvironmental conditions.